Whaaaat? You've generalized too much. What should I do to reproduce it?
Heh... Ok, more specific.
With two travel lanterns in inventory:
* find a low-lit interior
* place first one down
* place second one down next to first; first lantern will be illuminated by the second, but second lantern will not be illuminated by the first
* pick up first lantern and place back down; now second lantern will be illuminated by first, but just placed first lantern will not be illuminated
Leaving the cell and returning will show the proper lighting on both lights.
Addendum to description of bug:
This will continue to occur in that interior cell after each time one of the lights is picked up and placed back down.
However, it only happens the first time in an exterior - picking up and placing down the light after a subsequent load from any exterior cell will illuminate both lights properly.